checker: reorder Status with negatives for good, JSON as string

Make StatusUnknown the zero value (0) so an uninitialized CheckState
reads as "no signal yet" rather than as healthy. Push StatusOK and
StatusInfo to negative values so the natural int ordering matches
severity ordering: aggregators can simply take max() to compute the
worst status, and Unknown correctly sits above OK/Info but below Warn.

Status now (un)marshals as its string name ("OK", "WARN", ...) so the
wire format is stable across any future renumbering. UnmarshalJSON
still accepts raw ints for backward compatibility with older snapshots
and clients.

checker/types.go

@@ -155,15 +155,21 @@ type CheckerOptionsDocumentation struct {
 }
 
 // Status represents the result status of a check evaluation.
+//
+// Numeric ordering is severity ordering: lower = better, higher = worse.
+// StatusUnknown is intentionally the zero value, so an uninitialized
+// CheckState reads as "no signal yet" rather than as a healthy OK.
+// "Good" statuses are negative so that aggregators can simply take the
+// max() of a set of statuses to compute the worst one.
 type Status int
 
 const (
-	StatusUnknown Status = iota
-	StatusOK
-	StatusInfo
-	StatusWarn
-	StatusCrit
-	StatusError
+	StatusOK      Status = -2
+	StatusInfo    Status = -1
+	StatusUnknown Status = 0 // zero value: not initialized / no signal yet
+	StatusWarn    Status = 1
+	StatusCrit    Status = 2
+	StatusError   Status = 3
 )
 
 // String returns the human-readable name of the status.
@@ -186,6 +192,46 @@ func (s Status) String() string {
 	}
 }
 
+// MarshalJSON serializes Status as its string name so the wire format
+// is stable across any future reordering of the underlying int values.
+func (s Status) MarshalJSON() ([]byte, error) {
+	return json.Marshal(s.String())
+}
+
+// UnmarshalJSON accepts either the string name (preferred) or a raw int
+// (for backward compatibility with older clients/snapshots).
+func (s *Status) UnmarshalJSON(data []byte) error {
+	if len(data) > 0 && data[0] == '"' {
+		var name string
+		if err := json.Unmarshal(data, &name); err != nil {
+			return err
+		}
+		switch name {
+		case "OK":
+			*s = StatusOK
+		case "INFO":
+			*s = StatusInfo
+		case "UNKNOWN", "":
+			*s = StatusUnknown
+		case "WARN":
+			*s = StatusWarn
+		case "CRIT":
+			*s = StatusCrit
+		case "ERROR":
+			*s = StatusError
+		default:
+			return fmt.Errorf("unknown status %q", name)
+		}
+		return nil
+	}
+	var n int
+	if err := json.Unmarshal(data, &n); err != nil {
+		return err
+	}
+	*s = Status(n)
+	return nil
+}
+
 // CheckState is the result of evaluating a single rule.
 type CheckState struct {
 	Status  Status         `json:"status"`